Corrections for Exchange and Screening Effects in Low-energy Beta Decays

Abstract The beta spectra of 241 Pu and 63 Ni have been recently measured using metallic magnetic calorimeters. This powerful experimental technique allows theoretical beta spectra calculations to be tested at low energy with an accuracy never before achievable. Their comparison with classical beta calculations exhibits a significant deviation below 4 keV for 241 Pu and 8 keV for 63 Ni. The atomic exchange effect explains the main part of this deviation in the 63 Ni beta spectrum. This effect has a significant contribution, equivalent to the magnitude of the screening, in the 241 Pu beta spectrum.